[thread knowledge points] - spin lock

---

Preface

Spin lock is a basic knowledge point , Should be familiar with the master .

---

One 、 What is spin lock ？

The spin ” It can be understood as “ Self rotation ”, there “ rotate ” finger “ loop ”, such as while Loop or for loop .“ The spin ” Is that I keep cycling here , Until the goal is achieved . Not like ordinary locks , If you can't get the lock, you go into blocking .

The spin lock does not release CPU Time slice of , Instead, try to acquire the lock by spinning , If the attempt fails , Then keep trying , Until it is successfully obtained .

Two 、 Why use a spin lock ？

If the content of a critical zone is very “ Short ”, Then, if the lock cannot be obtained by using threads “ Blocking ” To lock , Then the cost of thread state switching is likely to be greater than the cost of executing the content of the critical area , Poor efficiency . If a thread has been found in the critical area “ Occupy ” Under the circumstances , Through a certain number of [ The spin ] To wait for the thread that has obtained the permission to use the critical area to release the lock , Then you can avoid frequent thread switching to a certain extent , To improve efficiency .

3、 ... and 、 Instance to explain

1.5 Start ,java.util.concurrent.atomic The atomic classes in the package basically have the shadow of spin lock .

AtomicInteger

    public final int getAndIncrement() {
    
        return unsafe.getAndAddInt(this, valueOffset, 1);
    }

Take a look at this self incrementally invoked method

    public final int getAndAddInt(Object var1, long var2, int var4) {
    
        int var5;
        do {
    
            var5 = this.getIntVolatile(var1, var2);
        } while(!this.compareAndSwapInt(var1, var2, var5, var5 + var4));
        return var5;
    }

there do while It's a spin operation , If you encounter other threads competing for modification during the modification process, resulting in your own modification failure , Just try to add one again in a dead cycle , Until we succeed .

Four 、 Implement a reentrant spin lock by yourself

TODO : ==''

5、 ... and 、 Disadvantages and adaptation scenarios

shortcoming

While avoiding the overhead caused by thread state switching , Introduced new overhead , Because spinlocks need to keep trying to get locks . If the lock has not been released , Or the lock takes too long , Will lead to useless attempts to spin , Wasted processor resources .

Applicable scenario

The use of spin lock needs to meet the following two conditions at the same time ：

1. The degree of concurrency is not high
2. The critical zone is short and uncomplicated

The above two conditions are actually to ensure that the spin can end as soon as possible , Avoid waste CPU resources .

---

summary

Spin lock can improve program efficiency , But we must pay attention to the use scenario , Otherwise it would be counterproductive .